> ... and maybe fortnight is not that
> frequent in formal (business) language?

We have almost no formal distinctions between formal and informal 
language. There is some distinction between polite and familiar forms 
of address but these tend to be informal ;-).

Mother / mum. mummy, ma
Father / Dad, daddy
Hello, Good morning / Hi.


While there are indeed "colloquialisms" and contractions which you 
would not use in polite or business settings, almost any word which is 
acceptable English is 'good to go' in almost any context. So 
'fortnight' could be used without raising an eyebrow in any context at 
all.


        Russell


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